Determination of Shrinkage Value of Wall Materials: Testing Standards, Methods, and Applications

The shrinkage value of wall materials is a decisive indicator of dimensional stability, directly governing the crack resistance and long-term durability of masonry and partition structures. Determination of the shrinkage value of wall materials involves standardized conditioning, precision length measurement, and calculation of linear drying shrinkage expressed in mm/m. This article explains the testing principles, specimen requirements, applicable standards such as GB/T 11969, GB/T 4111, and ASTM C426, typical acceptance limits, and practical applications of shrinkage testing in quality control and engineering projects.

Technical Overview

Shrinkage of wall materials refers to the irreversible reduction in linear dimensions that occurs as hydration products mature and as absorbed moisture leaves the pore structure of the material after installation. When blocks, panels, or boards with a high shrinkage potential are used in walls, restrained drying shrinkage generates tensile stress within the masonry. Because wall materials have limited tensile strength, this stress frequently manifests as visible cracking along mortar joints, step-pattern cracks at wall openings, or separation cracks at the interface between walls and structural frames. Such defects impair appearance, reduce sound insulation and fire performance, and create pathways for water ingress.

The shrinkage value is therefore a mandatory factory-control and type-test parameter for most categories of wall materials. It is defined as the change in length of a conditioned specimen per unit of original gauge length, customarily expressed in millimeters per meter (mm/m). Accredited third-party laboratories perform this determination under national and international standards to verify product conformity, support certification, and provide engineers with input data for crack-control design such as control joint spacing and reinforcement detailing.

Key standards governing the determination of shrinkage value of wall materials include:

• GB/T 11969, Test Methods of Autoclaved Aerated Concrete, which specifies the drying shrinkage test for AAC blocks and panels.
• GB/T 11968, Autoclaved Aerated Concrete Blocks, which defines the acceptance limits for shrinkage of AAC products.
• GB/T 4111, Test Methods for Concrete Small Hollow Blocks, covering shrinkage measurement of concrete masonry units.
• ASTM C426, Standard Test Method for Linear Drying Shrinkage of Concrete Masonry Units, used widely in North America and international markets.
• ASTM C596 and related EN methods for drying shrinkage of mortars and renderers applied to wall systems.

Testing Samples

Autoclaved Aerated Concrete (AAC) Blocks and Panels

AAC is among the most shrinkage-sensitive wall materials because of its highly porous, reaction-bonded microstructure. Testing typically uses prismatic specimens of 40 mm x 40 mm x 160 mm sawed from production units, fitted with gauge studs at both ends. Both the standard conditioning method (20 ± 2 °C, 43 ± 2 % relative humidity) and the rapid method (drying at 60 ± 5 °C) are applied, and the reported value is the mean of at least three specimens.

Concrete and Lightweight Aggregate Concrete Blocks

Dense concrete hollow blocks, lightweight aggregate concrete blocks, and fly-ash blocks are tested on prisms or full units in accordance with GB/T 4111 or ASTM C426. Specimens incorporate demountable mechanical gauge points or reference studs so that length changes can be tracked with a comparator or strain-gauge instrument.

Gypsum Wall Products

Gypsum partition boards and blocks are evaluated for linear shrinkage after complete desiccation, since calcium sulfate hemihydrate products undergo measurable dimensional change during the transition to the dihydrate equilibrium state and subsequent drying.

Composite and Panel-Type Wall Materials

Cement-based sandwich panels, fiber-reinforced panels, and other prefabricated wall elements are sampled as representative prisms or coupon sections containing all functional layers, ensuring the measured shrinkage reflects the behavior of the finished product rather than a single constituent.

Testing Items

The scope of a wall material shrinkage determination program generally includes the following measured and derived quantities:

• Drying shrinkage value, expressed in mm/m, from the reference conditioned state to the equilibrium dried state.
• Length change as a function of time, used to construct the shrinkage development curve and identify the point of equilibrium.
• Moisture content at the reference state and at equilibrium, required to interpret shrinkage in relation to the material moisture characteristic.
• Mass loss during conditioning, confirming that drying has reached the constant-mass criterion.
• Wetting expansion or irreversible shrinkage, where the residual deformation after re-saturation is of engineering interest.
• Compliance evaluation against the maximum shrinkage limits stated in the relevant product standards.

Testing Methods

Specimen Preparation and Conditioning

Specimens are cut to the prescribed dimensions and gauge studs are cemented to the end faces with rapid-setting adhesive. Specimens are then brought to the reference moisture state. For AAC under GB/T 11969, the reference condition is reached either by equilibrating at 20 ± 2 °C and 43 ± 2 % relative humidity over a saturated salt solution, or by the rapid procedure of oven conditioning at 60 ± 5 °C. Under ASTM C426, concrete masonry units are first water-saturated to establish the wet reference length, after which they are air-dried in a controlled environment until the length change between successive readings falls below the specified convergence threshold.

Length Measurement

Lengths are measured with a length comparator or digital displacement gauge with a resolution of 0.001 mm or better. The instrument is zeroed against a reference Invar or quartz gauge bar of the same nominal gauge length before every reading session to eliminate thermal drift. Specimens are handled at the gauge points only, and readings are taken after the specimens have stabilized at the measurement temperature. Measurements are repeated at defined intervals until the difference between consecutive readings satisfies the constant-length criterion of the standard.

Calculation

The drying shrinkage value is calculated as:

S = (L1 - L2) / (L0 - 2H) x 1000

where S is the drying shrinkage value in mm/m, L1 is the mean initial length reading in mm, L2 is the mean final length reading at equilibrium in mm, L0 is the overall nominal specimen length in mm, and H is the embedded depth of each gauge stud in mm. The result for each specimen is reported, and the arithmetic mean of the valid specimens, rounded to 0.01 mm/m, represents the shrinkage value of the sample.

Acceptance Criteria

For autoclaved aerated concrete blocks conforming to GB/T 11968, the drying shrinkage value shall not exceed 0.50 mm/m when determined by the standard method at 43 % relative humidity, and shall not exceed 0.80 mm/m when determined by the rapid 60 °C drying method. Concrete masonry units under ASTM C426 and corresponding product specifications are assessed against the linear shrinkage limits declared by the manufacturer or required by the project specification. Exceeding these limits classifies the lot as nonconforming, and the material may be rejected or restricted to non-critical applications.

Applications

Determination of the shrinkage value of wall materials serves multiple purposes across the product life cycle:

• Factory quality control, where shrinkage trending is used to monitor autoclave cycles, mix proportion stability, and curing uniformity.
• Type testing and product certification, providing the documented evidence required for conformity marking and market access.
• Construction engineering, where measured shrinkage values inform joint spacing, the placement of control joints, mesh reinforcement in plaster, and flexible connection detailing at wall-to-frame interfaces.
• Dispute resolution and failure diagnosis, comparing the as-delivered material against standard limits when cracking occurs in service.
• Research and development of new formulations, comparing shrinkage performance of alternative binders, aggregates, and curing regimes.
• Import and export inspection, where accredited third-party laboratories issue test reports recognized by regulators and buyers.

FAQ

What is an acceptable shrinkage value for autoclaved aerated concrete blocks?

Under GB/T 11968, AAC blocks must exhibit a drying shrinkage value of no more than 0.50 mm/m by the standard method (20 ± 2 °C, 43 ± 2 % RH) or no more than 0.80 mm/m by the rapid method (drying at 60 ± 5 °C). Project specifications may impose stricter limits in crack-sensitive applications.

How long does a shrinkage determination take?

The rapid method, which relies on 60 °C drying, typically reaches equilibrium within several days to about one week. The standard method at ambient temperature and controlled humidity may require three to four weeks because moisture equilibration of porous wall materials is diffusion-limited.

Why are two conditioning methods specified, and do they give the same result?

The standard humidity method reproduces realistic service conditions, while the accelerated oven method shortens the test period for factory control. Because the drying history and final equilibrium moisture differ, the methods yield systematically different values, which is why separate limits of 0.50 mm/m and 0.80 mm/m apply and why the conditioning method must always be stated on the test report.

What equipment is required for the test?

Essential equipment includes a length comparator with 0.001 mm resolution, an Invar reference bar for calibration, precision dial or digital displacement gauges, a temperature- and humidity-controlled conditioning chamber or drying oven with 60 ± 5 °C capability, a balance for constant-mass verification, and gauge studs with rapid-setting adhesive for specimen instrumentation.

How does excessive shrinkage translate into wall cracking?

When a wall panel is restrained by mortar joints, ties, or an adjacent frame, its tendency to shorten under drying generates internal tension. If the accumulated shrinkage strain exceeds the strain capacity of the material or the mortar interface, cracks initiate, typically at corners of openings and along bed joints. Limiting the shrinkage value at the material acceptance stage is therefore the primary preventive measure against in-service cracking.

Can shrinkage results from different standards be compared directly?

Direct comparison is not valid without correction, because standards differ in reference moisture state, conditioning path, specimen geometry, and gauge length. Accredited third-party laboratories report the governing standard together with the result, and specifiers should always verify that the tested condition matches the acceptance basis referenced in the product standard or project specification.